Effects of atmospheric powder on microstructure and piezoelectric properties of PMZN-PZT quaternary ceramics

The effects of atmospheric powder on microstructure and piezoelectric properties of Pb(Mn_1/3Nb_2/3)O₃–Pb(Zn_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMZN-PZT) quaternary ceramics were investigated. Specimens with various contents of Pb(Mn_1/3Nb_2/3)O₃ from 0 to 20 mol% were prepared by columbite two-stage process with and without atmospheric powder of PbZrO₃. The results revealed that the atmospheric powder is favored to the liquid-state sintering process in PbO vapor pressure equilibrium. The specimen sintered with atmospheric powder is homogenous and the fracture is intergranular. However, the specimen sintered without atmospheric powder is less homogenous and the fracture is essentially transgranular. On the other hand, the pyrochlore phase was formed along with the perovskite phase for the specimens sintered without atmospheric powder and the second phase was seriously detrimental to the electromechanical properties. The superior piezoelectric properties were observed for the specimens sintered with atmospheric powder. By optimizing the specimen composition, excellent piezoelectric, and dielectric properties (Q_m=2528, K_p=0.55, tan δ=0.003) were obtained at 10 mol% Pb(Mn_1/3Nb_2/3)O₃.     

Reduced temperature-dependent thermal conductivity of magnetite thin films by controlling film thickness

We report on the out-of-plane thermal conductivities of epitaxial Fe₃O₄ thin films with thicknesses of 100, 300, and 400 nm, prepared using pulsed laser deposition (PLD) on SiO₂/Si substrates. The four-point probe three-omega (3-ω) method was used for thermal conductivity measurements of the Fe₃O₄ thin films in the temperature range of 20 to 300 K. By measuring the temperature-dependent thermal characteristics of the Fe₃O₄ thin films, we realized that their thermal conductivities significantly decreased with decreasing grain size and thickness of the films. The out-of-plane thermal conductivities of the Fe₃O₄ films were found to be in the range of 0.52 to 3.51 W/m · K at 300 K. For 100-nm film, we found that the thermal conductivity was as low as approximately 0.52 W/m · K, which was 1.7 to 11.5 order of magnitude lower than the thermal conductivity of bulk material at 300 K. Furthermore, we calculated the temperature dependence of the thermal conductivity of these Fe₃O₄ films using a simple theoretical Callaway model for comparison with the experimental data. We found that the Callaway model predictions agree reasonably with the experimental data. We then noticed that the thin film-based oxide materials could be efficient thermoelectric materials to achieve high performance in thermoelectric devices.     

Injection molding of surface modified powders with high solid loadings: A case for fabrication of translucent alumina ceramics

Solid loading is a critical key to the fabrication of ceramic compacts with high densities via ceramic injection molding. As reported in most previous work, solid loading of ultra-fine alumina feedstock system could be achieved only up to ∼58 vol% with stearic acid (SA) as the surface modification agent. In present work, different from the traditional work in which SA has been introduced just in the powder blending process, we have successfully prepared the feedstock with a much higher solid loading up to ∼64 vol% by a prior ball milling treatment of ceramic powders with a small amount of SA before the traditional blending process. It can be attributed to that SA can be coated homogeneously around the powder surfaces by a chemical reaction induced by ball milling treatment. Highly translucent Al₂O₃ ceramics have been fabricated, which suggests an alternative route for fabrication of translucent ceramics with high quality.     

水热一步法合成SO42-/Zr-PHTS及其催化性能

在低酸度硫酸体系中,以P123(EO₂₀PO₇₀EO₂₀)为模板剂水热法一步合成硫酸化锆掺杂PHTS固体酸催化剂(SO₄^2-/Zr-PHTS),利用XRD、TEM、N₂吸附-脱附以及NH₃-TPD、Py-FTIR等手段对其进行表征。结果表明,SO₄^2-/Zr-PHTS具有有序的六方相介孔结构,具有以L酸为主的弱、中强度的酸性中心;随着硅锆摩尔比的增加,其酸量逐渐减少,比表面积逐渐增加,孔容、孔径无显著变化。SO₄^2-/Zr-PHTS在催化四氢呋喃聚合中表现出良好的反应性能,推测固体酸的表面酸性和孔结构决定着催化四氢呋喃聚合的反应性能。     

Thermoelectric properties of Ca0.8Dy0.2MnO3 synthesized by solution combustion process

High-quality Ca_0.8Dy_0.2MnO₃ nano-powders were synthesized by the solution combustion process. The size of the synthesized Ca_0.8Dy_0.2MnO₃ powders was approximately 23 nm. The green pellets were sintered at 1150-1300°C at a step size of 50°C. Sintered Ca_0.8Dy_0.2MnO₃ bodies crystallized in the perovskite structure with an orthorhombic symmetry. The sintering temperature did not affect the Seebeck coefficient, but significantly affected the electrical conductivity. The electrical conductivity of Ca_0.8Dy_0.2MnO₃ increased with increasing temperature, indicating a semiconducting behavior. The absolute value of the Seebeck coefficient gradually increased with an increase in temperature. The highest power factor (3.7 × 10^-5 Wm^-1 K^-2 at 800°C) was obtained for Ca_0.8Dy_0.2MnO₃ sintered at 1,250°C. In this study, we investigated the microstructure and thermoelectric properties of Ca_0.8Dy_0.2MnO₃, depending on sintering temperature.     

Microstructure and ionic conductivity properties of gadolinia doped ceria (GdxCe1−xO2−x/2) electrolytes for intermediate temperature SOFCs prepared by the polyol method

Gd_0.1Ce_0.9O_1.95 and Gd_0.2Ce_0.8O_1.9 powders were prepared through the polyol process without using any protective agent. Microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and impedance analysis methods. The results of the thermogravimetry/differential thermal analysis (TG/DTA) and XRD indicated that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The XRD patterns of the samples revealed that the crystallite size of the samples increased as calcination temperatures increased. The sintering behavior and ionic conductivity of pellets prepared from gadolinia doped ceria (GDC) powders, which were calcined at 500 °C, were also investigated. The relative densities of the pellets, which were sintered at temperatures above 1300 °C, were higher than 95%. The results of the impedance spectroscopy revealed that the GDC-20 sample that was sintered at 1400 °C exhibited an ionic conductivity of 3.25×10⁻² S cm⁻¹ at 800 °C in air. This result clearly indicates that GDC powder with adequate ionic conductivity can be prepared through the polyol process at low temperatures.     

Al-doped ZnO varistors prepared by a two-step doping process

ABSTRACT

It is difficult to dope Al into main grains of ZnO varistor ceramics, especially for small doping amount. Generally, all raw materials including Al dopant are directly mixed together and sintered into ceramics. However, in this direct doping process, Al is apt to stay in grain boundaries, and almost does not enter grains. This does harm to the electrical properties of ZnO varistors. In this paper, we proposed a two-step doping process. Al₂O₃ powder was first mixed only with a part of the ZnO powder and pre-sintered. The pre-sintered powder was mixed with other additives such as Bi₂O₃ and the rest ZnO. Then ZnO varistor ceramics were prepared via solid state sintering processes. Results showed that two-step doped ZnO varistors exhibited improved electrical properties with a significant increased nonlinear coefficient and a great decreased leakage current compared to directly doped ones because more Al was incorporated into ZnO grains.     

Preparation and heat-insulating property of the bio-inspired ZrO2 fibers based on the silk template

Natural silk fibers were used as the template to prepare biomorphic ZrO₂ fibers. Silk fibers were first immersed into a Zr(NO₃)₄ solution and then sintered in air at high temperatures to produce the final ZrO₂ fibers. Their microstructures, phases, synthesis process, infrared absorption spectra and thermal conductivity were analyzed. The results show that these synthesized fibers retained the morphologies of silk faithfully. These ZrO₂ fibers also obtained the ability of absorbing infrared from the silk, so that they possessed better heat-insulating property than the traditional ZrO₂ fibers.     

Effect of sintering temperature on microstructure and transport properties of Li3xLa2/3−xTiO3 with different lithium contents

Li_3xLa_2/3−xTiO₃ (LLTO) powder with different lithium contents (nominal 3x = 0.03–0.75) was synthesized via a simple sol–gel route and then calcination of gel-derived precursor at 900 °C which was much below the calcination temperature required for synthesizing the LLTO powder via solid state reaction route. The LLTO powder of sub-micron sized particles, derived from such sol–gel method, showed almost no aggregation. Starting from the sol–gel-derived powder, the LLTO ceramics with different lithium contents were prepared at different sintering temperatures of 1250 and 1350 °C. It demonstrated that our sol–gel route is quite simple and convenient compared to the previous sol–gel method and requires lower temperature for the LLTO. Our results also illustrated that lithium content significantly affects the structure and ionic conductivity of the LLTO ceramics. The dependence of the ionic conductivity on the lithium content, lattice structure, microstructure and sintering temperature was investigated systematically.     

Fabrication of magnesium aluminate (MgAl2O4) spinel foams

This paper reports on a novel-processing route for fabricating magnesium aluminate (MgAl₂O₄) spinel (MAS) foams from aqueous suspensions containing 30–35 vol.% solids loading. A stoichiometric MAS powder formed from alumina (71.8%) and magnesia (28.2%) at 1400 °C was surface passivated against hydrolysis in an ethanol solution of H₃PO₄ and Al(H₂PO₄)₃ at 80 °C for 24 h. Stable aqueous suspensions with 30–35 vol.% solids loading were prepared using the surface passivated MAS powder with the help of tetra-methylammonium hydroxide (TMAH) and an ammonium salt of polyacrylic acid (Duramax D-3005) employed as dispersing agents. An aqueous solution of N-cetyl-N,N,N-trimethylammonium bromide (CTMAB) was utilized to create foam in aqueous MAS suspensions by mechanical frothing. Liquid foam was then consolidated in non-porous moulds by introducing a polymerization initiator and a catalyst under ambient conditions. Dried (at >90 °C for 24 h) MAS foams were then sintered for 1 h at 1650 °C. For comparison purposes, dense MAS bodies out of an un-passivated stoichiometric MAS powder, and, dense as well as foams out of alumina were also prepared in this study. The sintered properties of MAS and alumina ceramics were characterized by various means and thus obtained results are presented and discussed in this paper. The sintered MAS foams exhibited a porosity of about 74–76% and a compressive strength of about 4–7.2 MPa inline to values reported for other ceramic foams in the literature.     

Synthesis and thermoelectric performance of Ta doped Sr0.9La0.1TiO3 ceramics

Ceramics samples of Sr_0.9La_0.1Ti_1−xTa_xO₃ have been synthesized by conventional solid-state reaction method. X-ray powder diffraction characterization indicates that all samples are of single phase with cubic symmetry. The high-temperature electrical resistivity decreases with the increasing of tantalum content except for x = 0.05 sample. Negative Seebeck coefficients have been obtained for all samples, which means conduction mechanism being n-type. The absolute Seebeck coefficient decreases with the increase of tantalum concentration. The power factor decreases with increasing of tantalum substitution. Small amount tantalum doping can reduce the thermal conductivity. The lowest thermal conductivity obtained is 2.9 W/mK for x = 0.03 at 1074 K. The highest thermoelectric figure of merit still observes in Sr_0.9La_0.1TiO₃, reaches 0.29 at 1046 K, which is a relatively higher value in n-type oxide thermoelectric materials.     

Effect of Cu powder addition on thermoelectric properties of Cu/TiO2−x composites

Spark plasma sintering was used to fabricate Cu/TiO_2−x composites by adding Cu powder to nonstoichiometric titanium dioxide, TiO_2−x. The composition and crystal forms of the composites were examined. The thermoelectric properties of the composites were measured and the effects of composite formation on these properties were discussed. The rule of mixture (ROM) of composite and general effective medium theory (GEM) were used to investigate the composite effects of the Cu/TiO_2−x composites. The results revealed that the electrical resistivities of the composites was much lower than that of TiO_2−x. As the added amount of Cu powder increased, the electrical properties of the composites shifted from semiconductor behavior to metallic behavior. The thermoelectric performances of the composites improved as a result of composition formation. The thermoelectric performance can be improved by adjusting the balance among electrical resistivity, thermal conductivity and the Seebeck coefficient, based on the composite effects.     

Thermal shock resistance of in situ formed Si2N2O–Si3N4 composites by gelcasting

Thermal shock resistance of Si₂N₂O–Si₃N₄ composites was evaluated by water quenching and subsequent three-point bending tests of strength diminution. Si₂N₂O–Si₃N₄ composites which was prepared with in situ liquid pressureless sintering process using Yb₂O₃ and Al₂O₃ powders as sintering additives by gelcasting showed no macroscopic cracks and the critical temperature difference (ΔT_c) could be up to 1400 °C. A mass of pores existed in the sintered body and the irregular shaped fibers extended from the pores increased the thermal shock property.     

Controlling Thermophoresis in the Exothermic Nitridation of an Aluminum Aerosol

Thermophoretic deposition in an exothermic aluminum (AI) nitridation aerosol process was redirected from the reactor wall to a flowing inert powder when the inert powder was admixed with the feed atomized AI. The inert powder can be a compatible (aluminum nitride) or removable (carbon) powder, which provides for a lower-temperature surface to be in close proximity to the exothermic reaction sites. Aluminum nitride manufactured by this process was pressurelessly sintered with 3 wt% Y₂O₃ to fabricate dense AIN parts exhibiting high thermal conductivity (141 W/m – K).     

Synthesis of α-Al2O3 platelets and kinetics study for thermal decomposition of its precursor in molten salt

The precursor, ammonium aluminum carbonate hydroxide (AACH) was synthesized via solid-state reaction at 60 °C. The experimental results show that the AACH is orthorhombic NH₄Al(OH)₂CO₃, and the calcined powder products are hexagonal platelets of α-Al₂O₃ which can be obtained from calcining AACH in molten salt at 1000 °C. The linear and nonlinear methods were used to calculate the activation energies of the thermal process of AACH. The calculated results indicated the decomposition process involved two stages which were single-step kinetic processes. The most probable reaction mechanisms of the two stages were estimated by two comparative methods. The values of pre-exponential factor A of the two stages were obtained on the basis of E_α and the reaction mechanisms.     

Auto-ignition route to thermoelectric oxide NaxCo2O4 powder with high compactibility

The ultra fine powder of thermoelectric oxide Na_xCo₂O₄ has been synthesized via an auto-ignition route following with airflow shatter process, which has the flaky shape in different nominal x compositions. Single γ-phase Na_xCo₂O₄ crystal structure was obtained in varying Na contents of 1.4-1.8 at the calcining temperature of 1153 K. The airflow shatter process was found to be beneficial for obtaining uniform particles with smaller the average size and larger BET surface area. The compaction module and the nonlinear exponent of powder compaction are about 4.037 MPa and 4.368 calculated form Huang Peiyun's compacting equation, respectively, which reveals that the Na_xCo₂O₄ powder has high compactibility.     

Preparation of single-phase magnesium silicon nitride powder by a two-step process

Single-phase magnesium silicon nitride (MgSiN₂) powder was prepared by a two-step process: combustion synthesis followed by acid washing. The effects of starting material compositions, diluent addition and N₂ pressure on the phase compositions of the final products were studied, and the acid washing process was also discussed. It is difficult to synthesize single-phase MgSiN₂ by one-stage combustion reaction through regulating the process parameters because of the evaporative loss of Mg. Combustion synthesis of MgSiN₂ by using an excess of magnesium in the starting materials and then acid washing off the impurity was an effective way to prepare the single-phase MgSiN₂ powder.     

Fuel mixture approach for solution combustion synthesis of Ca3Al2O6 powders

Single-phase 3CaO·Al₂O₃ powders were prepared via solution combustion synthesis using a fuel mixture of urea and β-alanine. The concept of using this fuel mixture comes from the individual reactivity of calcium nitrate and aluminum nitrate with respect to urea and β-alanine. It was proved that urea is the optimum fuel for Al(NO₃)₃ whereas β-alanine is the most suitable fuel for Ca(NO₃)₂. X-ray diffraction and thermal analysis investigations revealed that heating at 300 °C the precursor mixture containing the desired metal nitrates, urea and β-alanine triggers a vigorous combustion reaction, which yields single-phase nanocrystalline 3CaO·Al₂O₃ powder (33.3 nm). In this case additional annealing was no longer required. The use of a single fuel failed to ensure the formation of 3CaO·Al₂O₃ directly from the combustion reaction. After annealing at 900 °C for 1 h, the powders obtained by using a single fuel (urea or β-alanine) developed a phase composition comprising of 3CaO·Al₂O₃, 12CaO·7Al₂O₃ and CaO.     

Novel conductive magnetic nanocomposite based on poly (indole-co-thiophene) as a hemoglobin diagnostic biosensor: Synthesis,characterization and physical properties

The novel conductive nanocomposite has been successfully prepared by emulsion polymerization. First, magnetite nanoparticles were synthesized via coprecipitation reaction. Then, poly (indole-co-thiophene)@Fe₃O₄ nanocomposite was prepared via emulsion copolymerization of indole and thiophene monomers using sodium dodecyl sulfate as an emulsifier and ammonium persulfate as an oxidant in the presence of Fe₃O₄ nanoparticles. Characterization of the synthesized copolymer, Poly (In-co-T), and its magnetic nanocomposite were studied by Fourier transform infrared spectra, X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, differential scanning calorimetric, UV-vis spectrophotometer, and vibrating sample magnetometer. Also, the electrical conductivity of copolymer and nanocomposite were determined by four-probe instrument. Results showed a synergic effect in thermal stability by good interaction between polymer chain and magnetic nanoparticles. The conductivity of the nanocomposite was higher than bare copolymer, and increase of nanoparticles content caused an increment in the conductivity of the nanocomposites. The applicable properties of proposed conductive nanocomposite as a base at electrochemical biosensing have been investigated.